In the hard copy technique using electrophotography, toners of lower fixing temperature and wider fixing region have been required in recent years for black toners for use in black and white printing in view of the demand for the reduction of energy (saving of power consumption) and high speed operation and simplification and stabilization of fixing systems.
Further, the hard copy technique has been rapidly developed to full color application and, particularly, market for full color products have been extended more and more. In color images by the full color electrophotography, all colors are reproduced generally by using color toners of three colors: yellow, magenta and cyan as three primary colors or using four colors by adding black color thereto. That is, full color images are formed by repeating the process of charging, exposure, development and transfer on every toners described above, forming toner images comprising toners of plural colors on recording media and fusing and mixing toner images (color mixing), thereby fixing them on the recording media.
In the color electrophotography that requires development for several times and super-position of several kinds of toner images of different colors on one identical support as a fixing step, fixing property of color toners is an extremely important factor. That is, a color toner to be fixed should have a wide color reproduction range capable of being fused till the boundary between the toner particles is eliminated thereby providing transparency so that the toner particles do not cause random reflection to light and hinder color reproduction. Further, appropriate glossiness or luster is necessary.
Fixing apparatuses used for fixing color toners use rollers made of excellent materials of good surface releasability, and most of them are coated with a great amount of oils on the surface of the rollers, etc. However, coating of a great amount of oils for improving the releasability results in oil stains on transfer paper, increase in the cost and requiring a space for containing oils to bring about problems, for example, of complicating the fixing apparatus or increasing the size thereof.
Oils are generally used for fixing the color toners because it is necessary to increase the heat fusibility more upon heat fixing and lowering the viscosity to provide transparency in black toners for black and white printing. However, toners using such a resin reduce the cohesive force upon heat melting to cause deposition of toners to a heat fixing roller thereby bringing about high temperature offset. Accordingly, in order to prevent the high temperature offset, it is customary to coat oils to the fixing rollers thereby decreasing the depositability of the toners to the fixing rollers.
Then, it has been required in recent years to cope with an oilless mechanism without using an offset preventing effect mechanism by silicon oils with an aim of simplifying the fixing system mechanisms and preventing undesired effects of oils on images (oil stains and stickiness). For this purpose, it has been attempted use of so-called oilless toners not coating oils to fixing rollers that require improvement for the fixing property from low temperature to high temperature, and dispersion of wax into the toner is generally proposed. However, in a case of color toners, since it is necessary that the wax properly exudes from the toner of reduced viscosity as described above, and it is difficult to prevent offset.
JP-B No. 32624/1990 discloses a toner containing a polyester resin and a releasing agent comprising carnauba wax and polyolefin wax. However, since this is a black toner for use in black and white printing, transparency of the toner and fusing compatibility between toner particles are not taken into consideration and this is not sufficient to have a wide fixing region as a color toner.
JP-A No. 2917/1999 discloses a color toner containing a mixture of a plant wax and a synthetic hydrocarbon wax but no appropriate exuding effect of wax from the resin is taken into consideration and the effect thereof is insufficient to provide a wide fixing region at high speed and oilless function.
JP-A No. 284528/2000 proposes to select an olefin polymer having a cyclic structure and high viscosity as a toner binder in order to prevent offset and, further, to use a wax selected from amide wax, carnauba wax, higher fatty acid and ester thereof, higher fatty acid metal soap, partially saponified higher fatty acid ester, higher fatty acid alcohol, polyolefin wax and paraffin wax. However, no appropriate exuding effect of respective waxes from the resins is taken into consideration and the non-offset region is insufficient and impractical.